Calcium transport in the cortical portion of the thick ascending limb of Henle's loop is being studied in isolated perfused segments of rabbit renal tubules in vitro. Radioisotopic flux studies reveal that a large component of the calcium transport is passive, secondary to transepithelial voltage. In addition, parathyroid hormone (PTH) has been shown to significantly increase calcium transport in this segment by a mechanism which has not yet been fully defined, but which does involve PTH-sensitive adenylate cyclase.